1. Technical Field
Embodiment of the present invention relates to an organic light-emitting diode (OLED) including a multi-layered hole transporting layer and a flat display device including the organic light-emitting diode. More particularly, embodiments of the present invention relate to an OLED including a hole transporting layer that includes a plurality of layers formed using a hole transporting material and a charge generation material which have different energy levels from each other, and to a flat display device including the OLED.
2. Description of the Related Art
Organic light emitting diodes (OLEDs) are self-emission devices that have wide viewing angles, high contrast ratios, short response times, and excellent brightness, driving voltage, and response speed characteristics. OLEDs also enable generation of multi-color images.
In a typical organic light-emitting diode, an anode is formed on a substrate, and a hole transport layer, emission layer, electron transport layer, and cathode are sequentially formed (in this order) on the anode. In this regard, the hole transport layer, the emission layer, and the electron transport layer are organic films including organic compounds. When a voltage is applied between the anode and the cathode, holes injected from the anode pass through the hole transport layer and migrate toward the emission layer, and electrons injected from the cathode pass through the electron transport layer and migrate toward the emission layer. The holes and electrons, which are carriers, are recombined in the emission layer to generate excitons, and then the excitons change from an excited state to a ground state, thereby generating light.
The hole transport material used in the hole transporting layer has, in general, good hole injection function or hole transport function, and thus a formed device has low driving voltage. That is, if a hole transport material having high hole mobility is used in the hole transporting layer, the driving voltage of the formed device is substantially decreased. However, charges are excessively injected, and thus a formed device may have low efficiency and a short lifetime. To resolve these problems, many efforts have been made.
For example, in an organic light-emitting diode including an anode and a cathode formed on a substrate, some research has been conducted into providing the organic light-emitting diode with a first hole transporting layer (P-HTL) doped with a P-type dopant and a second hole transporting layer (HTL) (which are sequentially formed in this order); at least one emission layer (EML) formed on the second HTL; and a first electron transporting layer (ETL) and a second ETL (N-ETL) doped with an N-type dopant (which are sequentially formed on the EML in this order), where the EML has a quantum-well structure. This organic light-emitting diode is a PIN-structure phosphorescent organic light-emitting diode, and the organic layers included in the organic light-emitting diode include organic thin films doped with P-type and N-type dopants, and the emission layer is formed as a quantum-well structure to reduce an energy barrier to trap recombined excitons therein, thereby improving luminescent characteristics. However, in a phosphorescent organic light-emitting diode having a PIN structure, the thermal stability of an organic material is relatively reduced, and thus the organic light-emitting diode has a reduced lifetime. Also, because (unlike a typical inorganic semiconductor) the phosphorescent organic light-emitting diode having the PIN structure does not have crystallinity, it is difficult to reproduce current control.
Additionally, research has been conducted into providing an organic EL device including a cathode, an anode, and a HIL, a HTL, an EML, an ETL, an EIL between the cathode and the anode, where the HIL is formed by doping an electron accepting impurity on a hole injection layer material, and the ionization potential (Ip(HIL)) of the hole injection layer material of the hole injection layer, the ionization potential (Ip(HTL)) of the hole transporting layer material, and the ionization potential(Ip(EML)) of the emission layer material comply with the relationship: Ip(EML)>Ip(HTL)≧Ip(HIL)≧Ip(EML)−0.4 eV. In the HIL included in the organic EL device, an electron accepting dopant (p-type doping) is added to, for example, a material having a triarylamine moiety structure, a carbazole moiety structure, and an oxadiazole moiety structure, and thus the hole injection properties with respect to the EML are improved, and luminescent efficiency of the organic EL device is maintained or improved, thereby decreasing the driving voltage. However, owing to the inclusion of the HIL doped with the p-type dopant and thus injection of excess holes, the organic EL device has reduced lifetime and efficiency.
Further research has been conducted into an organic EL device including a cathode layer; an anode layer facing the cathode layer; and an EML including an organic compound between the cathode layer and the anode layer, where when holes are injected through the cathode layer and electrons are injected through the anode layer, an excitation state occurs in the organic compound of the EML and thus light is generated. Also, in this organic EL device, an electron accepting material is positioned between the cathode layer and the anode layer, is provided to at least one HTL that transports the holes injected through the cathode layer, and is located in a portion that is not adjacent to the cathode layer. This organic EL device includes two or more layered HTLs located adjacent to the cathode layer, a hole transport molecule that constitutes the HTL, an electron accepting material layer, and an electron accepting material that constitutes the electron accepting material layer. In detail, the organic EL device may have a substrate/cathode layer/first hole transport layer/mixed layer including a first hole transport molecule and an electron accepting material/mixed layer including a second hole transport molecule and an electron accepting material/second hole transport layer/emission layer structure. However, in an organic EL device having a first hole transport layer/p-doped first hole transport layer/p-doped second hole transport layer/second hole transport layer structure, it is difficult to control the balance of charges formed in the EML.